In some communication networks, for example in outdoor lighting networks, each node device has two or more communication interfaces. In the example shown on FIG. 1, each node 100 of the network has a mesh interface 101 and a GPRS interface 102 for example. The GPRS interface 102 is used for performing the auto-commissioning with a backend, e.g., by means of UDP/DTLS/CoAP. In this sense, if the DTLS handshake is successful, the device can be registered and some configuration parameters can be sent to the device. Once all the node devices 100 have been commissioned, the mesh network can be configured and the networking parameters transmitted to a remote controller 110 over the GPRS connection. Furthermore, the GPRS module of some of the node devices 100 can be disabled so that those devices with a disabled GPRS talk to the backend only over the remaining devices that still have a GPRS interface enabled (e.g. node device 100a). Furthermore, this device 100a can act as a simple router further simplifying the overall communication architecture and ensuring end-to-end operation between the network and the remote controller 110 in the backend. This means that a part of the communication is done over the mesh network and another part over the GPRS network. The GPRS interface can also be another long range communication interface such as UMTS, LTE or even a long range sub-GHz radio with which the devices can communicate in a star topology in a long range.
In such a network, it is possible to have an all-IP system, thus minimizing the number of protocol translations. To this aim, 6LoWPAN is one of possible protocols that can be used. However, it is required to make sure that some basic communication patterns are still feasible, namely:
1. Unicast communication from the backend to any node device: this can be used for switching on/off a luminaire (Downlink Unicast).
2. Unicast communication from any node device in the lighting system to the backend: this can be used for energy reporting (Uplink Unicast).
3. Multicast communication from the backend to a set of devices in the lighting system: this can be used for a software update or for switching on/off a group of luminaires (Downlink Multicast).
4. Local communication in the lighting system between a number of node devices: this can be used to enable products requiring Peer to Peer communication. In such products, detectors and wireless communication are used to provide light on demand, i.e. when a moving person or vehicle is detected.
In a 6LoWPAN network, the routing protocol is specified by Routing Protocol for Low-power and lossy Networks (RPL). A RPL-based network is composed by RPL instances. Each of these RPL instances can have one or more Destination Oriented Directed Acyclic Graphs (DODAGs). Each DODAG ends in a special node called root.
Two types of instances can be found in the node devices:                Global instances, which are identified by an instance id. Each global instance can include multiple DODAGs (with a different root on each).        Local instances, which are special instances associated to a node. Each local instance can only have one DODAG. Two fields are needed to identify them: an instance id and a DODAG id. The DODAG id is a unique and reachable IP address of the node which acts as the root of the DODAG.        
A node can only join one DODAG within an RPL instance, and communications between DODAGs of the same instance are not possible (they are isolated). On the other hand, a node can join different RPL instances at the same time. Thus, a node could own or be part of multiple local instances.
Moreover, there is a need in such network to support Multicast routing protocol. To achieve the above communication goals, it could be attempted to create multiple subnetworks each associated to a border router (with enabled GPRS), which is the root of the DODAGs of Global instance. However, this has the limitation that local communication is not possible when we look at the borders of the networks, as shown on FIG. 2. On this figure, it appears that subnetworks N1 and N2 are isolated and cannot communicate directly with this topology of the network. Communication between N1 and N2 can only be done via the backend, which in this case requires the use of GPRS transmission. Thus, communication from a node device from the subnetwork N1 to another node device from the subnetwork N2 is not efficient. Moreover, no multicast routing for nodes of both N1 and N2 is supported in this example.